The aforementioned patent also describes the fact that the duct is formed, in succession from the upstream end towards the downstream end, of a convergent inlet nozzle, a cylindrical part, and a divergent nozzle, the blades of the rotor rotating in the cylindrical part, and the support for the rotor and the rotational-drive mechanism within the duct being provided either by the vanes alone, or by substantially radial support arms, or finally, by arms and the vanes, still substantially radial ones. The aforementioned patent also describes a flow-straightener produced in the form of a monolithic assembly with a plurality of stationary vanes and mounted as a single piece within the divergent nozzle of the duct, in which nozzle it is fitted on the outlet side. Such a flow-straightener includes not only an annular body or an internal ring, but also a concentric external ring, housed in a recess in the divergent nozzle, and equipped with a flange for fastening against a lateral face of the fairing, the vanes being fastened radially between the internal and external rings, it being possible for the rings to exhibit cutouts for passage of the support arms, the vanes then being located between the cutouts.